Actually, I can't reproduce the problem. I just tested the following:
Two user accounts on a Win 7 machine, "fast user switching"
User 1, settings: allow multiple instances, don't run the servers
User 2, settings: allow multiple instances, run the servers
- "User 1" MC played audio to local speakers
- Another PC/MC instance played video from "User 2" MC Library Server.
- A third PC with a DLNA controller program (not MC) played audio from "User 2" MC DLNA Server.
All three tasks were running simultaneously without problems.
The MC version was 16.0.39, but I don't think this has radically changed from MC15.